SN is not painless. Well you might not have physical pain in the traditional sense of pain like a cut or a break but it can be extremely unpleasant. Vomiting, extreme heartrate, nausea, high anxiety, lightheadedness. This might be nothing for some people but it really freaks me out so everyone has to consider what their tolerance level is. SN is not just a cheaper N. N is anaesthetic and SN is a poison. There's a huge difference.
To answer your question any method is painless if you can make yourself go unconscious first. I'm trying to figure this out. When I figure out how to make myself go unconscious or at least extremely drowsy I'll either hang (can't do it consciously) or drown.